Dynamics and structure of mountain autochthonous spruce-beech forests: impact of hilltop phenomenon, air pollutants and climate

Mountain forests are strongly influenced by the extreme climate, short growing season and stress from environmental pollution and lower fertility of soils. The paper analyses the effect of the environment (climate and air pollutants) on the structure, production and dynamics of autochthonous spruce-beech forest stands in protected areas in the summit parts of the OrlickE hory Mts., Czech Republic. The spatial pattern of tree layer was random in lower parts below the summit and aggregated under the hilltop phenomenon on an extreme edaphic site, such as aggregated horizontal structure of natural regeneration. In most cases, the relationship between the spatial pattern of tree layer and natural regeneration was significantly negative (alpha = 0.05) at a smaller distance (from stem to 0.6-6.1 m) except stands under the strong hilltop phenomenon (positive effect to 2.1 m). The stand density ranged from 440 to 760 trees ha(-1) and the number of natural regeneration was 4 584-6 360 recruits ha(-1). Dominant height decreased with increasing influence of hilltop phenomenon (P < 0.001). The volume of live trees was 239-536 m(3) ha(-1). The radial growth of dominant European beech (Fagus sylvatica L.) indicated a relatively balanced long-term trend of tree-ring width in 1900-2014, but diameter increment of admixed Norway spruce (Picea abies /L./Karst.) after 1978 significantly decreased (P < 0.001) and since 1998 radial increment in spruce distinctly increased. Radial growth of spruce was significantly negatively correlated with mean SO2 and NOX concentrations, especially in April (P < 0.001), but there was no effect on radial growth of beech. Air pollution had a significantly higher negative effect on radial growth of spruce on the hilltop compared to the lower part of the hill . The correlation between radial increment and temperature was stronger than in precipitation for both species in mountain areas compared to lowlands. The hilltop phenomenon significantly influenced the structure of spruce-beech mountain forests. The lowest dynamics was observed in stands in middle slope parts compared to summit parts of the hill.